1. Field of the Invention
The present invention relates to a method and apparatus to measure the gain spectra of erbium doped fiber amplifiers. In particular, the invention relates to use of a filtered unpolarized amplified stimulated emission noise source in an apparatus to measure the gain spectra of erbium doped fiber amplifiers.
2. Description of Related Art
Undersea communication network systems and other fiber optic network systems require repeaters periodically spaced to compensate for attenuation in the signal transmission medium. Optical fiber networks include repeaters connected between links of optical fiber cable. Submarine optical fiber cables include one or more optical fibers and often include conductive wires (e.g., copper wires) to deliver power to the repeaters.
Most modern optical repeaters in lightwave transmission systems use optical amplifiers, and erbium doped fiber amplifiers (EDFA) have become the dominant technique for optical signal amplification. An EDFA includes a length of erbium doped fiber in which a lightwave propagates. An energy source or pump laser pumps the erbium doped fiber into an xe2x80x9cexcitedxe2x80x9d state. From this state, signal gain is produced through emission stimulated by signal light passing through the excited fiber and optical noise is produced through spontaneous decay of the excited state of the erbium doped fiber. The design of the fiber amplifier seeks to maximize the signal gain and minimize the optical noise produced while meeting the required power output and bandwidth specification.
The gain of an EDFA is characterized over its bandwidth. This gain may be shaped (via optical filters) to provide more uniform gain across all signal channels or wavelengths within the amplifier bandwidth.
To select a particular optical filter to shape the gain of the amplifier, it is essential to accurately measure the gain spectra of the EDFA across the signal bandwidth. A systematic error as small as 0.1 dB across the band will result in an accumulated error of 20 dB after 200 repeaters which yields unacceptable system performance.
Conventional test equipment uses a broadband wavelength laser source with plural discrete lasers. FIG. 1, illustrates a conventional multiple wavelength source 100 that includes plural discrete laser sources 102 the optical outputs of which are combined in one or more power combiners 104 via connectors 106 into a single piece-wise broadband source. Laser sources 102 may comprise, for example, 64 laser sources individually tuned to separate frequencies spaced on 0.7 nanometer centers (corresponding to 64 optical channels) used to cover a 43 nanometer bandwidth, associated with a conventional (xe2x80x9cCxe2x80x9d) band amplifier (i.e. from approximately 1526 through 1569 nanometers). The wavelength laser source 100 is typically coupled to an optical signal conditioning circuit used by amplifier test equipment.
The close spacing of the separate lasers in the source is needed to provide a piece wise continuous distribution over the bandwidth to accurately measure the gain spectrum in the presence of spectral hole burning associated with erbium doped fiber amplifiers. The spectral hole is stable over time, is of the order of 4 nanometers wide and cannot be ignored without resulting in an error when measuring the gain spectra of the EDFA. However, a drawback associated with this conventional test-set is that it uses separate laser sources for the corresponding number of optical channels when testing an EDFA.
Thus, it is an object of the present invention to provide a test method and apparatus to reduce the cost of measuring the gain spectra of optical amplifiers including erbium doped fiber (EDFAs) and Raman amplifier under test. It is another object to reduce the time and/or number of measurement required to test an EDFA. It is yet another object of the present invention to provide a method to improve the accuracy and wavelength resolution of gain spectra measurements.
These and other objects are achieved in a test apparatus that includes a broadband noise source, a test station, and a spectrum analyzer. These and other objects are further achieved by a method that uses the test apparatus to measure the spectral gain of an erbium doped fiber amplifier (EDFA) under test includes steps of providing an optical signal from a broadband noise source to an input of the EDFA under test, measuring a power at a test wavelength at an output of the EDFA under test, adjusting the test wavelength, and repeating the steps of measuring and adjusting for a predetermined number of times.